Polishing method and polishing liquid

ABSTRACT

Disclosed is a method for polishing a surface of a substrate containing Ru or a Ru compound in a surface region, said method comprising a polishing step with a polishing liquid containing tetravalent cerium ions. The polishing liquid is prepared by adding a compound having a tetravalent cerium ion or its solution to a solvent in or immediately before the polishing step of the substrate.

BACKGROUND OF THE INVENTION

The present invention relates to a polishing method adapted for themanufacture of a semiconductor device, etc., and a polishing liquid usedin the polishing method.

In recent years, researches and developments of various fine processingtechnologies are being conducted in the field of manufacturing asemiconductor device in accordance with progress in the density andfineness of the semiconductor device. Particularly, a CMP (ChemicalMechanical Polishing) technology is absolutely necessary for flatteningthe interlayer insulating film, for forming a plug, for forming a buriedmetal wiring, and for forming a buried element isolation.

Use of the CMP technology is also being tried in the processing of anelectrode for a capacitor. Particularly, it is considered very importantto establish a method utilizing the CPM technology in the manufacture ofDRAM or FRAM of the next era using a perovskite crystal for forming adielectric film. It should be noted in this connection that it isnecessary to select a noble metal or a perovskite type conductive oxidefor forming the lower electrode of a capacitor in view of thecompatibility of the lower electrode with the dielectric film. However,the noble metal and the perovskite type conductive oxide is chemicallystable in general, making it difficult to employ a wet etching or a dryetching for processing the lower electrode of the capacitor. Such beingthe situation, it is considered very important to establish a methodusing the CPM technology.

On the other hand, the possibility of processing is increased in the CMPtechnology because a chemical function and a mechanical function areutilized in good balance in the polishing by the CMP technology.

However, the conventional polishing liquid used in the CMP technologywas defective in that the polishing rate was low, leading to a lowmanufacturing efficiency of the semiconductor device. Also, theconventional polishing liquid was low in the selectivity ratio of thepolishing rate relative to the underlying stopper film, making itdifficult to obtain an uniform and stable processed configuration overthe entire surface region of a single wafer or among different wafers.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to provide a polishing method thatpermits a high polishing rate and a high selectivity ratio of thepolishing rate relative to the underlying layer in the case where anoble metal or a perovskite type conductive oxide is polished by a CMPtechnology.

Another object of the present invention is to provide a polishing liquidused in the polishing method of the present invention.

According to a first aspect of the present invention, there is provideda polishing method, comprising the step of polishing a surface of asubstrate containing Ru or a Ru compound in a surface region with apolishing liquid containing tetravalent cerium ions.

According to a second aspect of the present invention, there is provideda polishing method, comprising the step of polishing a surface of asubstrate containing Ru or a Ru compound in a surface region with apolishing liquid containing tetravalent cerium ions, wherein thepolishing liquid is prepared by adding a compound having a tetravalentcerium ion to a solvent in or immediately before the polishing step ofthe substrate.

Further, according to a third aspect of the present invention, there isprovided a polishing liquid for polishing a surface of a substratecontaining Ru or a Ru compound in a surface region, wherein thepolishing liquid contains tetravalent cerium ions and nitrate ions.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention, and together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIG. 1 is a graph showing the change with time in the Ru polishing ratein the polishing with a polishing liquid;

FIGS. 2A to 2C are cross sectional views collectively showing a methodof manufacturing a capacitor according to each of first and fourthembodiments of the present invention;

FIGS. 3A to 3C are cross sectional views collectively showing a methodof manufacturing a capacitor according to each of second and fifthembodiments of the present invention; and

FIGS. 4A to 4C are cross sectional views collectively showing a methodof manufacturing a capacitor according to each of third and sixthembodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a polishing method comprising the step ofpolishing a surface of a substrate containing Ru or a Ru compound in asurface region with a polishing liquid containing tetravalent ceriumions.

There are divalent, trivalent and tetravalent cerium ions. Among thesecerium ions, the tetravalent cerium ion alone produces a high oxidizingpower and exhibits excellent polishing properties relative to Ru or a Rucompound.

It is desirable for the polishing liquid containing tetravalent ceriumions to further contain nitrate ions. For example, the polishing liquidshould desirably contain cerium (IV) nitrate or diammonium cerium (IV)nitrate. To be more specific, the polishing liquid used in the presentinvention is prepared by dissolving cerium (IV) nitrate in a solvent, orby dissolving diammonium cerium (IV) nitrate in a solvent. Of course, anaqueous solution of cerium (IV) nitrate and an aqueous solution ofdiammonium cerium (IV) nitrate can be used as effective polishingliquids in the present invention.

In the case of polishing Ru using a solution of cerium (IV) nitrate as apolishing liquid, the concentration of cerium (IV) nitrate in thesolution should desirably be 0.75% by weight or more, preferably 0.75 to2% by weight, and most preferably 1 to 1.75% by weight.

In the case of polishing Ru using a solution of diammonium cerium (IV)nitrate as a polishing liquid, the concentration of diammonium cerium(IV) nitrate in the solution should desirably be 3% by weight or more,preferably 3 to 8% by weight, and most preferably 4 to 7% by weight.

The other cerium (IV) compounds that can be used in the presentinvention for preparing the polishing liquid also include, for example,cerium sulfate, Ce(NO₃)₃OH, M₂Ce(NO₃)₆, and M₄Ce(SO₄)₄ (M: monovalentmetal ion).

The polishing liquid of the present invention produces a markedly highoxidizing power and a polishing capability, which are unimaginable inthe past, when applied to the polishing of a Ru layer or a Ru compoundlayer. Therefore, the polishing liquid produces its effect sufficientlyeven if abrasive grains are not contained in the polishing liquid. Whereabrasive grains are contained in the polishing liquid, the abrasivegrains include the grains of alumina, silica, ceria, and the like. Theabrasive grains should desirably be contained in the polishing liquid inan amount of 0.1 to 2% by weight.

In general, a cerium (IV) compound is stable where the compound iscontained in a solution in a high concentration. If the solution isdiluted to lower the concentration of the cerium (IV) compound, however,the oxidizing power and polishing capability of the compound are loweredwith time. It follows that, in the case of using a polishing liquidcontaining a cerium (IV) compound for the polishing treatment, thepolishing liquid must be used for the polishing soon after the dilution.Preferably, the polishing liquid should be used for the polishingimmediately after or simultaneously with the dilution.

FIG. 1 is a graph showing the change with time in the Ru polishing ratein the polishing with a polishing liquid containing 5% by weight ofdiammonium cerium (IV) nitrate, which was prepared by diluting withwater an aqueous solution containing 20% by weight of diammonium cerium(IV) nitrate. As apparent from FIG. 1, the polishing rate of Ru islowered with time.

It should be noted that an allowable fluctuation in the polishing rateis about 10%. FIG. 1 indicates that, if it is allowable for thepolishing rate to be lowered from 1 to 0.9, it is necessary to use thepolishing liquid for the polishing treatment within 8 hours from thepreparation.

As described above, it is most desirable to use the polishing liquidcontaining a cerium (IV) compound for the polishing treatmentsimultaneously with the dilution. For example, it is most desirable tosupply a compound having a tetravalent cerium ion to the substratesurface together with a solvent in polishing the substrate surface.

A typical example of the Ru compound that is to be polished in thepresent invention is SrRuO₃.

As described above, a polishing liquid containing tetravalent ceriumions is used in the present invention for polishing a Ru layer or a Rucompound layer so as to markedly improve the polishing rate. Theparticular technique of the present invention also permits markedlyimproving a ratio (selectivity ratio) of the polishing rate of a Rulayer or a Ru compound layer to the polishing rate of a SiO₂ layer.

Various embodiments of the present invention will now be described withreference to the accompanying drawings.

EMBODIMENT 1

How to manufacture a capacitor by the CMP technology will now bedescribed as a first embodiment of the present invention with referenceto FIGS. 2A to 2C.

As shown in the drawings, a plug 11 for electrical connection is buriedin an insulating film 12 formed on a main surface of a silicon substrate(not shown). Then, a SiO₂ film 13 is formed in a thickness of about 100nm by a plasma CVD method using TEOS on the entire surface, followed byforming an aperture 14 (opening) having a diameter of about 300 nm inthe SiO₂ film 13 right above the plug 11. After formation of theaperture 14, a Ru film 15 forming a lower electrode of a capacitor isformed on the entire surface by a sputtering method or a CVD method suchthat the aperture 14 is filled with the Ru film 15, as shown in FIG. 2A.

In the next step, the Ru film 15 is polished by a CMP method with theSiO₂ film 13 used as a stopper, with the result that the lower electrodeconsisting of the Ru film 15 is insulated for each cell, as shown inFIG. 2B. Used in the CMP method is a polishing liquid prepared by addingdiammonium cerium nitrate to a dispersion prepared by dispersing aluminaparticles having a particle diameter of 30 nm in water. It is desirablefor the polishing liquid to contain 1% by weight of alumina particlesand 5% by weight (or 4 to 7% by weight) of diammonium cerium nitrate.Used in the CMP method is IC1000/Suba400 (trade name of a polishing padmanufactured by Rhodel Nitta K.K.). The polishing is performed under aload of 100 gf/cm², and the top ring and the turntable are rotated at aspeed of 100 rpm.

In the next step, a BaSrTiO₃ film 16 acting as a dielectric film of acapacitor is formed in a thickness of about 40 nm by a sputtering methodor a CVD method. Where the BaSrTiO₃ film 16 as formed is amorphous, thefilm is annealed to form crystals of perovskite structure. Then, a Rufilm 17 acting as an upper electrode of the capacitor is formed by asputtering method or a CVD method in a thickness of about 60 nm, asshown in FIG. 2C.

After formation of the Ru film 17, an interlayer insulating film (notshown) is formed on the Ru film 17, followed by selectively removing theinterlayer insulating film to form an opening. Further, a plug (notshown) for electric connection to the Ru film 17 is formed in theopening so as to finish preparation of the capacitor for DRAM of thenext era.

EMBODIMENT 2

How to manufacture a capacitor by the CMP technology will now bedescribed as a second embodiment of the present invention with referenceto FIGS. 3A to 3C.

As shown in the drawings, a plug 21 for electrical connection is buriedin an insulating film 22 formed on a main surface of a silicon substrate(not shown). Then, a SiO₂ film 23 is formed in a thickness of about 150nm by a plasma CVD method using TEOS on the entire surface, followed byforming an aperture 24 (opening) having a diameter of about 300 nm inthe SiO₂ film 23 right above the plug 21. After formation of theaperture 24, a Ru film 25 forming a lower electrode of a capacitor isformed on the entire surface by a sputtering method or a CVD method suchthat the aperture 24 is filled with the Ru film 25, as shown in FIG. 3A.

In the next step, the Ru film 25 is polished by a CMP method with theSiO₂ film 23 used as a stopper, with the result that the lower electrodeconsisting of the Ru film 25 is insulated for each cell, as shown inFIG. 3B. Used in the CMP method is a polishing liquid prepared by addingdiammonium cerium nitrate to a dispersion prepared by dispersing aluminaparticles having a particle diameter of 30 nm in water. It is desirablefor the polishing liquid to contain 1% by weight of alumina particlesand 5% by weight (or 4 to 7% by weight) of diammonium cerium nitrate.Used in the CMP method is IC1000/Suba400 referred to previously. Thepolishing is performed under a load of 100 gf/cm², and the top ring andthe turntable are rotated at a speed of 100 rpm.

Then, the SiO₂ film 23 is removed by a wet etching using hydrofluoricacid or ammonium fluoride or by a reactive ion etching using afluorocarbon-based gas, followed by forming a BaSrTiO₃ film 26 acting asa dielectric film of a capacitor by a sputtering method or a CVD method.Where the BaSrTiO₃ film 26 as formed is amorphous, the film is annealedto form crystals of perovskite structure. Then, a Ru film 27 acting asan upper electrode of the capacitor is formed by a sputtering method ora CVD method in a thickness of about 60 nm, as shown in FIG. 3C.

After formation of the Ru film 27, an interlayer insulating film (notshown) is formed on the Ru film 27, followed by selectively removing theinterlayer insulating film to form an opening. Further, a plug (notshown) for electric connection to the Ru film 27 is formed in theopening so as to finish preparation of the capacitor for DRAM of thenext era.

EMBODIMENT 3

How to manufacture a capacitor by the CMP technology will now bedescribed as a third embodiment of the present invention with referenceto FIGS. 4A to 4C.

As shown in the drawings, a plug 31 for electrical connection is buriedin an insulating film 32 formed on a main surface of a silicon substrate(not shown). Then, a SiO₂ film 33 is formed in a thickness of about 300nm by a plasma CVD method using TEOS on the entire surface, followed byforming an aperture 34 (opening) having a diameter of about 200 nm inthe SiO₂ film 33 right above the plug 31. It is desirable for the sidewall defining the aperture 34 to be inclined or tapered by about 10°relative to a vertical plane. After formation of the aperture 34, a Rufilm 35 forming a lower electrode of a capacitor is formed on the entiresurface by a sputtering method or a CVD method in a thickness of about60 nm. Further, a capping film 38 such as resist or SOG (Spin On Glass)is formed by, for example, a spin coating method on the entire surfaceto fill the aperture 34, as shown in FIG. 4A.

In the next step, the Ru film 35 and the capping film 38 are polished bya CMP method with the SiO₂ film 33 used as a stopper, with the resultthat the lower electrode consisting of the Ru film 35 is insulated foreach cell, as shown in FIG. 4B. Used in the CMP method is a polishingliquid prepared by adding diammonium cerium nitrate to a dispersionprepared by dispersing alumina particles having a particle diameter of30 nm in water. It is desirable for the polishing liquid to contain 1%by weight of alumina particles and 5% by weight (or 4 to 7% by weight)of diammonium cerium nitrate. Used in the CMP method is IC1000/Suba400referred to previously. The polishing is performed under a load of 100gf/cm², and the top ring and the turntable are rotated at a speed of 100rpm.

In the next step, the capping film 38 remaining within the aperture 34is removed, as shown in FIG. 4B. Where the capping film 38 is made ofresist, the capping film 38 is dipped in a peeling solution, or anashing is applied to the capping film 38 for removing the capping film38. Where the capping film 38 is made of SOG, it is effective to exposethe capping film 38 to an HF vapor for removing the capping film 38.Incidentally, the capping film 38 functions as a sacrificing film forpreventing the dust generated in the CMP step from being attached to theRu film 35 within the aperture 34.

Further, a BaSrTiO₃ film 36 acting as a dielectric film of a capacitoris formed in a thickness of about 40 nm by a sputtering method or a CVDmethod. Where the BaSrTiO₃ film 36 as formed is amorphous, the film isannealed to form crystals of perovskite structure. Then, a Ru film 37acting as an upper electrode of the capacitor is formed by a sputteringmethod or a CVD method in a thickness of about 60 nm, as shown in FIG.4C.

After formation of the Ru film 37, an interlayer insulating film (notshown) is formed on the Ru film 37, a followed by selectively removingthe interlayer insulating film to form an opening. Further, a plug (notshown) for electric connection to the Ru film 37 is formed in theopening so as to finish preparation of the capacitor for DRAM of thenext era.

Where the conventional polishing liquid was used for performing the CMPmethod, a ratio of the polishing rate of the Ru film to the polishingrate of the SiO₂ film, i.e., the selectivity ratio, was only about 2.Since the selectivity ratio was low, the SiO₂ film (13, 23, 33) failedto perform sufficiently the function of the stopper, making it difficultto control the thickness of the Ru film (15, 25, 35) after thepolishing. As a result, nonuniformity in the shape of the lowerelectrode was brought about in a single wafer or among different wafers,leading to a low reliability of the manufactured semiconductor device.

On the other hand, the polishing liquid defined in the present inventionpermits increasing the selectivity ratio of the Ru film to the SiO₂ filmto such a large value as 100, with the result that it is possible toobtain a stable processed shape. It should also be noted that thepolishing rate of the Ru film achieved by the conventional polishingliquid was only about 200 Å/min. On the other hand, the polishing liquidof the present invention permits increasing the polishing rate to 2000Å/min. Naturally, the CMP processing time for a single wafer can beshortened in the present invention so as to improve the manufacturingefficiency.

It should also be noted that, in the capacitor prepared by the CMPmethod of the present invention (particularly, embodiments 1 and 2), thesurface of the lower electrode in contact with the dielectric film ispolished microscopically smooth by the CMP method so as to moderate thecurrent concentration and to suppress the leak current. Further, thecrystallinity and degree of orientation of the dielectric film areimproved so as to increase the dielectric constant. As a result, theelectric characteristics and the reliability of the capacitor areimproved.

Table 1 shows the effectiveness of the polishing liquid of the presentinvention. Specifically, given in Table 1 are experimental data showinghow the polishing rate of the Ru film was changed depending on theoxidizing agent added to the polishing liquid. The polishing liquid usedin this experiment contained 1% by weight of alumina particles asabrasive grains.

TABLE 1 Dependence of Ru polishing rate on oxidizing agent Ru polishingSelectivity Oxidizing agent rate (Å/min) ratio to SiO₂ Diammonium cerium2000 100 nitrate (5 wt %) Hydrogen peroxide 200 10 (3.5 wt %) + nitricacid* Ammonium 50 2.5 persulfate (10 wt %) None 5 0.25 *Added to set pHat 2;

Table 1 shows that the polishing rate of the Ru film was drasticallyincreased to 2000 Å/min in the case of using diammonium cerium nitrateas the oxidizing agent. It is also shown that the selectivity ratio toSiO₂ is also increased by the increase in the polishing rate of the Rufilm. When it comes to a standard redox potential used as a criterion ofthe oxidizing power, diammonium cerium nitrate is inferior to ammoniumpersulfate. Specifically, the standard redox potential when thetetravalent cerium ion of diammonium cerium nitrate is converted intothe trivalent cerium ion is 1.72 V. On the other hand, the standardredox potential when the persulfate ion of ammonium persulfate isconverted into the sulfate ion is 2.01 V. Nevertheless, diammoniumcerium nitrate permits a greater polishing rate, which suggests thatdiammonium cerium nitrate causes Ru to perform a special reaction.

In embodiments 1 to 3 described above, alumina particles are containedas abrasive grains in the polishing liquid. Alternatively, silica orceria particles can be used as the abrasive grains in place of thealumina grains. It is also possible to use an aqueous solution itself ofdiammonium cerium nitrate, which does not contain abrasive grains, asthe polishing liquid. Further, the load, the rotation speeds of the topring and the turntable, etc. can be changed appropriately in thepolishing step.

EMBODIMENT 4

How to manufacture a capacitor by the CMP technology will now bedescribed as a fourth embodiment of the present invention with referenceto FIGS. 2A to 2C.

In embodiments 1 to 3 described previously, Ru was used for forming thelower electrode and the upper electrode of the capacitor. Incidentally,embodiments 4 to 6 that are to be described are substantially equal toembodiments 1 to 3, respectively, except that, in embodiments 4 to 6,SrRuO₃ is used in place of Ru for forming the lower electrode and theupper electrode of the capacitor. Such being the situation, embodiments4 to 6 will also be described with reference to FIGS. 2A to 4C,respectively.

As shown in FIGS. 2A to 2C, a plug 11 for electrical connection isburied in an insulating film 12 formed on a main surface of a siliconsubstrate (not shown). Then, a SiO₂ film 13 is formed in a thickness ofabout 100 nm by a plasma CVD method using TEOS on the entire surface,followed by forming an aperture 14 (opening) having a diameter of about300 nm in the SiO₂ film 13 right above the plug 11. After formation ofthe aperture 14, a SrRuO₃ film 15 forming a lower electrode of acapacitor is formed on the entire surface by a sputtering method or aCVD method such that the aperture 14 is filled with the SrRuO₃ film 15,as shown in FIG. 2A.

In the next step, the SrRuO₃ film 15 is polished by a CMP method withthe SiO₂ film 13 used as a stopper, with the result that the lowerelectrode consisting of the SrRuO₃ film 15 is insulated for each cell,as shown in FIG. 2B. An aqueous solution containing 1% by weight (or 1to 2% by weight) of diammonium cerium nitrate is used as the polishingliquid. Used in the CMP method is IC1000/Suba400 referred to previously.The polishing is performed under a load of 100 gf/cm², and the top ringand the turntable are rotated at a speed of 100 rpm.

In the next step, a BaSrTiO₃ film 16 acting as a dielectric film of acapacitor is formed in a thickness of about 40 nm by a sputtering methodor a CVD method.

Where the BaSrTiO₃ film 16 as formed is amorphous, the film is annealedto form crystals of perovskite structure. Then, a SrRuO₃ film 17 actingas an upper electrode of the capacitor is formed by a sputtering methodor a CVD method in a thickness of about 60 nm, as shown in FIG. 2C.

After formation of the SrRuO₃ film 17, an interlayer insulating film(not shown) is formed on the SrRuO₃ film 17, followed by selectivelyremoving the interlayer insulating film to form an opening. Further, aplug (not shown) for electric connection to the SrRuO₃ film 17 is formedin the opening so as to finish preparation of the capacitor for DRAM ofthe next era.

EMBODIMENT 5

How to manufacture a capacitor by the CMP technology will now bedescribed as a fifth embodiment of the present invention with referenceto FIGS. 3A to 3C.

As shown in the drawings, a plug 21 for electrical connection is buriedin an insulating film 22 formed on a main surface of a silicon substrate(not shown). Then, a SiO₂ film 23 is formed in a thickness of about 150nm by a plasma CVD method using TEOS on the entire surface, followed byforming an aperture 24 (opening) having a diameter of about 300 nm inthe SiO₂ film 23 right above the plug 21. After formation of theaperture 24, a SrRuO₃ film 25 forming a lower electrode of a capacitoris formed on the entire surface by a sputtering method or a CVD methodsuch that the aperture 24 is filled with the SrRuO₃ film 25, as shown inFIG. 3A.

In the next step, the SrRuO₃ film 25 is polished by a CMP method withthe SiO₂ film 23 used as a stopper, with the result that the lowerelectrode consisting of the SrRuO₃ film 25 is insulated for each cell,as shown in FIG. 3B. An aqueous solution containing 1% by weight (or 1to 2% by weight) of diammonium cerium nitrate, which does not containabrasive grains, is used as the polishing liquid in the CMP method. Usedin the CMP method is IC1000/Suba400 referred to previously. Thepolishing is performed under a load of 100 gf/cm², and the top ring andthe turntable are rotated at a speed of 100 rpm.

Then, the SiO₂ film 23 is removed by a wet etching using hydrofluoricacid or ammonium fluoride or by a reactive ion etching using afluorocarbon-based gas, followed by forming a BaSrTiO₃ film 26 acting asa dielectric film of a capacitor by a sputtering method or a CVD method.Where the BaSrTiO₃ film 26 as formed is amorphous, the film is annealedto form crystals of perovskite structure. Then, a SrRuO₃ film 27 actingas an upper electrode of the capacitor is formed by a sputtering methodor a CVD method in a thickness of about 60 nm, as shown in FIG. 3C.

After formation of the SrRuO₃ film 27, an interlayer insulating film(not shown) is formed on the SrRuO₃ film 27, followed by selectivelyremoving the interlayer insulating film to form an opening. Further, aplug (not shown) for electric connection to the SrRuO₃ film 27 is formedin the opening so as to finish preparation of the capacitor for DRAM ofthe next era.

EMBODIMENT 6

How to manufacture a capacitor by the CMP technology will now bedescribed as a sixth embodiment of the present invention with referenceto FIGS. 4A to 4C.

As shown in the drawings, a plug 31 for electrical connection is buriedin an insulating film 32 formed on a main surface of a silicon substrate(not shown). Then, a SiO₂ film 33 is formed in a thickness of about 300nm by a plasma CVD method using TEOS on the entire surface, followed byforming an aperture 34 (opening) having a diameter of about 200 nm inthe SiO₂ film 33 right above the plug 31. It is desirable for the sidewall defining the aperture 34 to be inclined or tapered by about 100relative to a vertical plane. After formation of the aperture 34, aSrRuO₃ film 35 forming a lower electrode of a capacitor is formed on theentire surface by a sputtering method or a CVD method in a thickness ofabout 60 nm. Further, a capping film 38 such as resist or SOG (Spin OnGlass) is formed by, for example, a spin coating method on the entiresurface to fill the aperture 34, as shown in FIG. 4A.

In the next step, the SrRuO₃ film 35 and the capping film 38 arepolished by a CMP method with the SiO₂ film 33 used as a stopper, withthe result that the lower electrode consisting of the SrRuO₃ film 35 isinsulated for each cell, as shown in FIG. 4B. An aqueous solutioncontaining 1% by weight (or 1 to 2% by weight) of diammonium ceriumnitrate, which does not contain abrasive grains, is used as thepolishing liquid in the CMP method. Used in the CMP method isIC1000/Suba400 referred to previously. The polishing is performed undera load of 100 gf/cm², and the top ring and the turntable are rotated ata speed of 100 rpm.

In the next step, the capping film 38 remaining within the aperture 34is removed, as shown in FIG. 4B. Where the capping film 38 is made ofresist, the capping film 38 is dipped in a peeling solution, or anashing is applied to the capping film 38 for removing the capping film38. Where the capping film 38 is made of SOG, it is effective to exposethe capping film 38 to an HF vapor for removing the capping film 38.Incidentally, the capping film 38 functions as a sacrificing film forpreventing the dust generated in the CMP step from being attached to theSrRuO₃ film 35 within the aperture 34.

Further, a BaSrTiO₃ film 36 acting as a dielectric film of a capacitoris formed in a thickness of about 40 nm by a sputtering method or a CVDmethod. Where the BaSrTiO₃ film 36 as formed is amorphous, the film isannealed to form crystals of perovskite structure. Then, a SrRuO₃ film37 acting as an upper electrode of the capacitor is formed by asputtering method or a CVD method in a thickness of about 60 nm, asshown in FIG. 4C.

After formation of the SrRuO₃ film 37, an interlayer insulating film(not shown) is formed on the SrRuO₃ film 37, followed by selectivelyremoving the interlayer insulating film to form an opening. Further, aplug (not shown) for electric connection to the SrRuO₃ film 37 is formedin the opening so as to finish preparation of the capacitor for DRAM ofthe next era.

Where the conventional polishing liquid was used for performing the CMPmethod, it was difficult to achieve a ratio, which was greater than 1,of the polishing rate of the Ru film to the polishing rate of the SiO₂film, i.e., the selectivity ratio. Since the selectivity ratio was low,the SiO₂ film (13, 23, 33) failed to perform sufficiently the functionof the stopper, making it difficult to control the thickness of theSrRuO₃ film (15, 25, 35) after the polishing. As a result, nonuniformityin the shape of the lower electrode was brought about in a single waferor among different wafers, leading to a low reliability of themanufactured semiconductor device.

On the other hand, the polishing liquid defined in the present inventionpermits increasing the selectivity ratio of the SrRuO₃ film to the SiO₂film to such a large value as 150, with the result that it is possibleto obtain a stable processed shape. It should also be noted that thepolishing rate of the SrRuO₃ film achieved by the polishing liquid ofthe present invention was as high as 3000 Å/min. Naturally, the CMPprocessing time for a single wafer can be shortened in the presentinvention so as to improve the manufacturing efficiency.

It should also be noted that, in the capacitor prepared by the CMPmethod of the present invention (particularly, embodiments 4 and 5), thesurface of the lower electrode in contact with the dielectric film ispolished microscopically smooth by the CMP method so as to moderate thecurrent concentration and to suppress the leak current. Further, thecrystallinity and degree of orientation of the dielectric film areimproved so as to increase the dielectric constant. As a result, theelectric characteristics and the reliability of the capacitor areimproved.

Table 2 shows the effectiveness of the polishing liquid of the presentinvention. Specifically, given Table 2 are experimental data showing howthe polishing rate of the SrRuO₃ film was changed depending on theoxidizing agent added to the polishing liquid. Abrasive grains were notcontained in the polishing liquid used in this experiment.

TABLE 2 Dependence of SrRuO₃ film SrRuO₃ polishing rate Oxidizing agent(Å/min) Diammonium cerium nitrate 3000 (1 wt %) Ammonium persulfate 280(10 wt %) Hydrogen peroxide 0 (3.5 wt %)

Table 2 shows that the polishing rate of the SrRuO₃ film was drasticallyincreased to 3000 Å/min in the case of using diammonium cerium nitrateas the oxidizing agent. When it comes to a standard redox potential usedas a criterion of the oxidizing power, diammonium cerium nitrate wasfound to be inferior to ammonium persulfate, as already pointed out.Nevertheless, diammonium cerium nitrate permits a greater polishingrate, which suggests that diammonium cerium nitrate causes SrRuO₃ toperform a special reaction.

In embodiments 4 to 6 described above, an aqueous solution of diammoniumcerium nitrate was used as the polishing liquid. Of course, it ispossible for the polishing liquid to contain abrasive grains such asalumina, silica or ceria particles. Further, the load, the rotationspeeds of the top ring and the turntable, etc. can be changedappropriately in the polishing step.

In embodiments 1 to 6 described above, Ru or SrRuO₃ was used for formingthe upper electrode of a capacitor. However, other materials such asRuO₂, W and WN can also be used for forming the upper electrode. Also,perovskite crystals such as SrTiO₃, BaTiO₃, PbTiO₃ and PbZrTiO₃ can alsobe used for forming the dielectric film in addition to BaSrTiO₃ used inthe embodiments described above. Where perovskite crystals exhibiting aferroelectricity such as BaSrTiO₃, BaTiO₃, PbTiO₃ and PbZrTiO₃ are usedfor forming the dielectric film, the dielectric film can also be appliedto an FRAM.

The present invention is not limited to the embodiments described above.In other words, the present invention can be worked in variouslymodified fashions within the technical scope of the present invention.

In the present invention, a polishing liquid containing diammoniumcerium nitrate is used in the CMP method so as to markedly increase thepolishing rate of a Ru film or a Ru compound film. It is also possibleto markedly increase a ratio of the polishing rate of the Ru or Rucompound film to the polishing rate of a SiO₂ film.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A polishing method comprising: preparing a first polishing liquidcontaining tetravalent cerium ions in a first concentration; adding asolvent for dilution to said first polishing liquid to form a secondpolishing liquid containing tetravalent cerium ions in a secondconcentration lower than the first concentration; and polishing asurface of a substrate containing Ru or a Ru compound in a surfaceregion with the second polishing liquid, wherein said Ru compound isSrRuO₃, and further wherein said addition of the solvent is carried outupon or immediately before the polishing of said substrate.
 2. Apolishing method according to claim 1, wherein said second polishingliquid does not contain abrasive grains.
 3. A polishing method accordingto claim 1, wherein said second polishing liquid contains cerium (IV)nitrate in a concentration of 0.75% or more by weight.
 4. A polishingmethod according to claim 3, wherein said second polishing liquidcontains cerium (IV) nitrate in a concentration of 0.75 to 2% by weight.5. A polishing method according to claim 1, wherein said secondpolishing liquid contains diammonium cerium (IV) nitrate in aconcentration of 3% or more by weight.
 6. A polishing method accordingto claim 5, wherein said second polishing liquid contains diammoniumcerium (IV) nitrate in a concentration of 3 to 8% by weight.
 7. Apolishing method according to claim 1, wherein said solvent has aproperty of dissolving a solute of said first polishing liquid and doesnot substantially contain any solute.
 8. A polishing method according toclaim 1, wherein said solvent consists essentially of water.